1. Field of the Invention
The present invention relates to a method for the production of tertiary olefins. More particularly, it relates to a method for the production of pure tertiary olefins by the decomposition of alkyl tert-alkyl ethers in the presence of a new and improved catalyst based on an alkaline earth exchanged faujasite.
2. Description of Related Art
Olefins, particularly tertiary olefins, may be commercially produced by the sulfuric acid extraction of such olefins from mixtures containing them obtained e.g., by steam cracking of petroleum feeds. Since this method uses sulfuric acid of high concentration, the use of expensive materials in the fabrication of the extraction apparatus is essential. Also, dilution of the acid to promote olefin recovery and reconcentrating the acid prior to recycling are required and are expensive. In addition, this method is not always advantageous industrially because tertiary olefins are subject to side reactions such as polymerization, hydration and the like during extraction with concentrated sulfuric acid.
It is also known that tertiary olefins may be prepared by reacting them selectively from such feeds with a primary alcohol in the presence of a acid catalyst to produce the corresponding alkyl tert-alkyl ethers. The tert-alkyl ethers are primarily formed, since the secondary olefins react very slowly and the primary olefins are completely inert. Such alkyl tert-alkyl ethers may then be easily separated and subsequently decomposed back to the tertiary olefins and the primary alcohol.
For producing tertiary olefins from alkyl tert-alkyl ethers, there have been proposed methods using various catalysts: For example aluminum compounds supported on silica or other carriers (U.S. Pat. No. 4,398,051); phosphoric acid on various supports (U.S. Pat. No. 4,320,232) ; and metal containing weakly acidic components on a carrier of &gt;20 M.sup.2 /gm surface area (British Pat. No. 1,173,128). In addition, inferior results are disclosed as being obtained utilizing carriers alone in the decomposition of methyl tertiary butyl ether (U.S. Pat. No. 4,398,051) and utilizing H.sub.2 SO.sub.4 treated clay in the decomposition of t-alkyl ether alkanols (U.S. Pat. No. 4,254,290).
One of the main disadvantages of such processes is that the disclosed catalysts do not have good catalyst life in that higher and higher temperatures, which eventually become limiting, are required to maintain high conversion of the alkyl tert-alkyl ethers. Additionally, larger amounts of the dialkyl ether by-product are produced as the catalyst ages with the disadvantage of a reduction in yield of the desired tertiary olefin. This lack of good catalyst life may be due to the instability of the catalyst, to high temperatures being required for good conversion thus promoting fouling, to the catalyst itself promoting fouling or to any or all of these. Also, a number of the catalysts such as ion exchange resins cannot be regenerated after use.
More recently, processes have been discovered which provide improved yields of tertiary olefin product. For example, U.S. Pat. No. 4,691,073 discloses a process for preparing tertiary olefins from alkyl tertiary alkyl ethers comprising contacting the ether with a catalyst which has been prepared by reacting a clay with HF and/or HCl and calcining the resultant clay product. Although the process produces very high yields and selectivity towards the production of tertiary olefin products, these catalysts often tend to become deactivated as a consequence of coke and/or polymeric build up in a relatively short on-stream time. Also, the aluminosilicate structure of many clays is not sufficiently stable to withstand repeated high temperature regenerations required to remove catalyst deposits.
Natural and synthetic faujasite catalysts are known for use in the conversion or pyrolysis of ethers and alcohols into olefins or distillate range hydrocarbons. For example, U.S. Pat. No. 4,467,133 discloses the conversion of methanol into a distillate range hydrocarbon mixture by passing methanol over a rare earth exchanged faujasite (such as zeolite X or Y) at a temperature below 6000.degree. F. U.S. Pat. No. 4,544,793 discloses a similar process using an exchanged aluminosilicate catalyst which has a specific X-ray diffraction pattern as shown in Table 1 of the patent.